When you discuss boundaries with your partner, there's some give and take. A "rule" that keeps getting broken can be looked at two ways. From one perspective, it was inconsiderate of your partner to break the rule you agreed on. From another perspective, if the rule is so impossible to follow, maybe it was a stupid/unfair/unrealistic rule.
A rule like "no dating monogamous people" is an example of a rule that can be difficult to follow. First of all, depending on where you live, there might not be a whole selection of out poly people to choose from. We become interested in people because of their unique qualities and shared interests, not necessarily because they are the right sex and are poly and happen to be around. You know?
Also, "non-monogamous" is an action, not a species of human. Non-monogamous people don't have tags on their ears. You can switch back and forth from being monogamous to being non-monogamous. You say you've been interested in poly for 10 years, but officially in a poly relationship for 1 year. So there must have been times in your life when you were monogamous, right? And now you're not.
So if your partner likes someone and they get along, but this someone has only had monogamous partnerships before, there's probably nothing wrong with her talking to them anyway, explaining the situation (that she's in another relationship, and if they got involved it wouldn't be exclusive). Then the other person could decide whether the arrangement could work for them or not.
"We should know as much as possible about a potential date beforehand" sounds vague. Beforehand to what? The first date? The first time you have sex? And how much is *possible* to find out about someone before you've dated them? Dating is usually the process of getting to know someone. You don't want to give your date a questionnaire where they fill in their allergies, shoe size, etc. But at the same time I agree that getting into bed with a stranger is probably a bad idea. So there's definitely some wiggle room in this "rule". Perhaps you and your partner weren't clear on what this "rule" meant.
I'll have to say that me and my husband only have one rule: that we use protection with outside partners. Other than that, we don't have rules, but we do try to be considerate of the other's feelings. That means checking in a lot and being reasonable.